What is Oxygen Saturation (SpO2)

Oxygen saturation is defined as the ratio of oxyhemoglobin to the total concentration of hemoglobin present in the blood (ie Oxyhemoglobin + reduced hemoglobin).

Hemoglobin is a iron-containing protein bound to red blood cells and makes up nearly all the oxygen presence (there is a minute amount dissolved in the plasma). Hemoglobin is responsible for transporting oxygen from lungs to other parts of the body, where the oxygen can be used by other cells. Oxyhemoglobin (HbO2) is the bright red hemoglobin that is a combination of hemoglobin and oxygen from the lungs.

A hemoglobin molecule can carry a maximum of four oxygen molecules. 1000 haemoglobin molecules can carry a maximum of 4000 oxygen molecules; if they together were carrying 3600 oxygen molecules, then the oxygen saturation level would be (3600/4000)*100 or 90%.

When arterial oxyhemoglobin saturation is measured by an arterial blood gas it is called SaO2. When arterial oxyhemoglobin saturation is measured non-invasively by pulse oximetry, it is called SpO2.

 

Reference

1. Oxygen saturation
2. Haemoglobin saturation graph
3. Definition and notation of hemoglobin oxygen saturation